A piezoelectric material generates electrical charge in response to a stress, which is termed the piezoelectric effect. The electrical charge produced is generally proportional to the applied force, changes sign between tensional or compressional stress, and determined by the piezoelectric coefficient. A larger piezoelectric coefficient is usually desirable in order to obtain a larger electric signal.
Piezoelectric sensors provide an electrical response to pressure, and act as pressure transducers. Piezoelectric sensors include acoustic sensors (such as noise, vibration, sound and ultrasound sensors), along with sonar devices, imaging devices, and the like. Pressure signals may be transmitted through a fluid from a monitored medium to a piezoelectric element of a sensor.
Piezoelectric crystals also exhibit dimension changes in response to electrical potentials, the inverse piezoelectric effect. Hence, piezoelectric devices include piezoelectric sensors, using the piezoelectric effect, and actuators that use the inverse piezoelectric effect.
If a piezoelectric material within a sensor has a low electrical resistivity, the generated charge rapidly drains away and electronic detection of the charge is therefore compromised. This is a problem for low frequency or static measurements. Hence, a high electrical resistivity is desirable. The length of time the charge is maintained is proportional to an RC time constant of the piezoelectric element (typically a piezoelectric material sandwiched between first and second electrodes). The minimum useful frequency of a piezoelectric sensor, the lower limiting frequency fLL, is inversely proportional to the time constant. Below fLL, the electrical charge drains away before it can be efficiently detected. Using improved materials with a low fLL, the sensor dynamic bandwidth can be extended into audio frequencies. Therefore, a large time constant is desirable for many applications.
Piezoelectric sensors include static sensors, and acoustic sensors that are used to monitor sound waves, vibration, and noise signals. Piezoelectric sensors have been used at normal ambient temperatures, and commonly include the piezoelectric material lithium niobate, (LiNbO3). However, single crystal lithium niobate has a limited operating temperature of 650° C. because of the low resistivity, restricting uses of such sensors (T. R. Shrout et al. Piezoelectric Materials in Devices, Ed. N. Setter, Ceramics Laboratory, EPFL, 2002, p. 413). Another commonly used piezoelectric material is the mineral tourmaline. However, this is relatively expensive, cannot easily be synthesized, and the piezoelectric coefficient is relatively low. Hence, there is a need for high temperature piezoelectric materials for improved piezoelectric sensors that can operate at temperatures above 650° C.